Payloads, such as satellites or spacecraft, which are mounted on launch vehicles, are subject to severe vibrations during flight. These vibrations are induced by multiple sources from liftoff to the instant of final separation from the launch vehicle. The dynamic mechanisms include ignition and operation of the rocket engines, transient vectoring forces at the nozzles, separation of rocket stages, aerodynamic effects and acoustic phenomena. The vibrations are often associated with severe quasi-static loads caused by axial thrust. The frequency content of the vibrations generally extends from 10-20 Hz to several kHz. The amplitude of the vibrations tends to be more severe in certain frequency bands and this is usually a function of the type of rocket motor being used. There is one class of commonly used solid rocket motors that generates high vibrations in the 50-60 Hz range.
A direct result of the severe vibrations generally experienced by payloads is that fatigue damage and failure can be incurred by sensitive payload components. Extensive engineering effort is normally expended to insure that this phenomenon is fully understood and avoided.
The mounting of the payload to the launch vehicle is usually done by attaching the lower spacecraft interface to the forward end of the rocket. Thus, the payload is normally cantilevered at the front of the launch vehicle. This configuration leads to axial components of the interface forces between the payload and vehicle even in the presence of purely lateral loads. These components are additive to those caused by the axial loads and vibrations. This observation underlines the importance of the axial load transfer at the interface and a great deal of attention has traditionally been given to this.
A rigid connection at the payload/vehicle interface has been widely used in the past, especially for vehicles with very robust payloads. In situations where a few sensitive components are to be used in the payload, these components are sometimes attached using individual vibration isolation mount systems. This approach is not cost or weight efficient for a fragile payload and the concept of complete payload isolation is now widely adopted in such situations.
Complete payload vibration isolation schemes generally use a flexible payload/vehicle interface. When the natural frequency of the payload vibrating on the flexibility of the interface is significantly lower than the frequency of the vibrations being transmitted through the vehicle to the interface, the payload is essentially isolated. The real challenge in the design of a satisfactory complete payload vibration isolation system is to satisfy two competing requirements. First, a payload mounting frequency low enough to achieve good isolation is required. Secondly, it is important to avoid the problematic interaction of the low frequency payload modes of vibration with the low frequency primary bending modes of the vehicle. The first requirement drives the payload frequency down while the second drives it up.
The difficulty of satisfying the two competing requirements is best understood with reference to specific example frequencies. In cases where it is desirable to isolate 55 Hz vibrations the axial payload mode must be less than 39 Hz to get any attenuation at all. A frequency of about 25 Hz would be desirable, as this would achieve a vibration transmissibility of only approximately 27%. The 25 Hz value is a lower bound of the acceptable frequency range because of interaction problems with rocket axial modes of vibration for a broad range of launch vehicle designs. Therefore, for these rockets, the payload isolation frequency in the axial mode of vibration should be in the 25-39 Hz range.
The lateral modes of vibration of the payload make the frequency requirements even more difficult to satisfy. Problematic interaction with bending modes of the rocket leads to a common requirement that the payload lateral mode of vibration should be greater than approximately 15 Hz. The lateral and axial modes of vibration of the payload are generally closely coupled for typical adapter designs. It is noted that if discrete springs are used to introduce flexibility into the payload mounting system it is difficult to avoid having the bounce mode higher than three times the lateral frequency. This ratio of three is applicable for geometries where the mounting circle has a diameter approximately equal to the height of the payload center of gravity above this circle. Thus, a 15 Hz lateral mode may well be associated with an axial frequency of 45 Hz which is clearly too high to achieve attenuation of the 50-60 Hz vibrations. These vibrations may well be amplified for such a system rendering the concept of discrete-spring vibration isolation infeasible for the frequencies cited.
A parameter that has significant bearing on the performance of a vibration isolation system is damping. Typically the higher the damping the greater is the vibration attenuation. It is therefore desirable to incorporate damping features into the design of an isolation system.
Accordingly, there is a need for payload mounting adapter that avoids the high axial-to-lateral frequency ratio inherent in the commonly used discrete spring isolation concepts, provides a convenient and integral mounting scheme without resorting to add-on devices such as springs or flexures, and leads to cost and weight savings.